17 Using Mixed Starter Cultures for Thai Nham

Pairote Wiriyacharee

Nham is traditionally made from fresh lean pork that is trimmed;
minced; mixed thoroughly with salt, potassium nitrate, cooked rice and
seasonings; and packed in either banana leaves (1) or cylindrical plastic bags
(2). Nham production in Thailand depends on chance contamination with wild
organisms - lactic acid bacteria and nitrate reducing bacteria. It is a long
process; generally the fermentation lasts 3 to 5 days depending on the season.
When nham is packed into cylindrical plastic bags, which exclude air, and is
held in the bags during fermentation, a microenvironment is selected for
microorganisms that are not only salt tolerant but can also grow in the absence
of air. In these gram-positive fermentative types of microorganisms, lactic acid
bacteria are predominant (3,4). The fermentable carbohydrates are used by those
organisms to produce organic acids, mainly lactic acid, that contribute to a
variety of flavors and textures. The nham finally develops approximately 1.0
percent total acidity as lactic acid and the pH is 4.3 (5).

MARKETING PROBLEMS

Problems in marketing traditional nham include its short shelf
life and high price and the intensive labor required for its production. It has
high energy costs if kept under refrigeration in the marketplace. Additionally,
the manufacturers have a heavy exposure to risk of losing a large stock through
a process failure. Pork meat is quite expensive, and the raw material cost is
increasing more quickly than the selling price. In addition, large-scale
production of nham has the problem of its short storage life. A longer shelf
life is required so that the nham can be distributed to the marketplace.
Therefore, the nham market needs the product to have consistent quality, safety,
and longer shelf life. The nham should stay fresh and not turn rancid or develop
an off flavor or change in color when it is in the marketplace.

On the other hand, nham production depends on natural
fermentation; the product quality therefore varies from batch to batch. The
shelf life of nham is quite short - approximately a week at Thai ambient
temperatures. Chilled conditions can extend the shelf life, but normally the
product is stored at ambient temperatures. The sanitation conditions of the
processing are also poor because of a lack of knowledge and technology. The
initial native lactic acid bacteria may be insufficient to bring about the
normal ripening process. This may allow pathogenic bacteria to grow before
lactic acid bacteria occur, resulting in the possibility of food poisoning.
Since most nham is consumed without further cooking, proper fermentation is of
paramount importance in ensuring the product's safety.

Somathiti (6) found that the initial coliform count was high in
nham - approximately 107 cells per gram - and decreased to 102 cells per gram on
the fifth day. An investigation of Salmonella in nham in the Bangkok market
showed that it was present in 56 (or 12 percent) of 450 samples. In nham
produced in Chiang Mai, Chiang Rai, and Ubonratchathani, Salmonella was found in
25 percent, 42 percent, and 11 percent, respectively, of the total samples.
However, Shigella sp. was not found in nham bought from any of these markets.

Thus, the nham process needs to be studied to improve product
quality, to give a more uniform standard quality, and to develop the technology
for applying of the process on an industrial scale before launching extensively
in the Thai and export markets.

NHAM DEVELOPMENT

In developing of an improved nham process, not only is there a
need for the knowledge of modern scientific discoveries and technological
developments but also the knowledge of consumers' needs and wishes. The final
product must be acceptable to consumers. A unified system is required that
combines scientific and consumer information for systematic development of the
nham product.

Effect of Starter Cultures

In our research mixed starter cultures and the carbon sources
used in nham formulation were important factors in determining product quality
(7). The starter cultures had a potential to make a good nham quality. Cooked
rice, a carbon source for lactic acid production by starter cultures, was an
important factor in nham fermentation.

The addition of L. plantarum to the nham mass accelerated very
distinctly the decrease in the pH of nham. Consequently, the firmness and color
developed, influenced directly by acid production. Those findings were in
agreement with the work of many researchers (8-12). P. cerevisiae increased the
firmness later during the last period of fermentation. The optimum growth of P.
cerevisiae is at pH 5.0 (13), the conditions during this period allow good
growth and acid production causing the increase in firmness. L. plantarum
inoculation had a very distinct effect in terms of firmness development when it
was used together with P. cerevisiae.

M. varians in the nham system significantly reduced nitrate to
nitrite during the initial fermentation and increased the tristimulus values at
the beginning of fermentation. L. plantarum then continued to intensify the
color. This finding agreed with the work of Deibel et al. (14); they reported
that nitrate-reducing activity generally occurred during the first 2 to 16
hours, while acid production was initiated after 8 to 16 hours. It was clear
that it was important to ensure the nitrate reducing activity of the M. varians
that took place prior to its inhibition by the growth of lactic acid bacteria.
The nitrite formed was decomposed spontaneously in acid surroundings into nitric
oxide, which subsequently reacted with myoglobin to form a pink compound -
nitrosomyoglobin. So the residual nitrate in the nham system reduced quickly
when acid was produced. The rate of nitrosomyoglobin formation increased with
falling pH, and this reaction takes place best in the pH range of 5.0 to 5.5
(15) and was therefore accelerated by L. plantarum. The L. plantarum inoculation
had a very distinct effect in terms of color development when it was used
together with M. varians.

On the other hand, L. brevis seemed to be a poor lactic acid
producer and decreased the color of the product and also produced gas, which
decreased the firmness of the nham.

Microbiological Quality

The starter cultures L. plantarum and P. cerevisiae increased
during the initial fermentation and were highest on the third day of
fermentation with 106 to 109 cfu/g-' (colony forming units) and then decreased
slowly during the later period of fermentation. In the nham sample, on the other
hand, the M. varians decreased during the fermentation approximately 2 log
cycles by the third day. The total bacterial count was related to the starter
cultures counts, but there was a little higher count of approximately 1 log
cycle. No yeasts or molds were detected in the finished nham.

The pathogenic bacteria, including Enterobacteriaceae and
Staphylococcus aureus, decreased during fermentation. In the nham fermented for
3 days the Enterobacteriaceae and 5. aureus counts were 102 and 103 cfu/g-',
respectively.

FERMENTATION DEVELOPMENT

In Thailand large amounts of cooked rice are added to the raw
nham mixture. It is degraded only slowly and may result in growth of undesirable
organisms during fermentation, particularly at high ripening temperatures.
Glucose is therefore added to the cooked rice. This ensures a sufficiently rapid
initial growth and nitrate reduction by M. varians and rapid later pH drop,
without inhibiting the chemical reactions necessary for the development of
firmness and desired color.

Cooked rice and glucose had no effect on pH reduction during
nham fermentation. As the fermentation time increased, the pH decreased. The pH
dropped rapidly after 18 hours of fermentation at 30°C, 43 percent relative
humidity with pH 5.1. The beginning of cooked rice reduction coincided with the
increase in reducing sugars after 12 hours of fermentation. The reducing sugars
declined after another 12 hours of fermentation, and this coincided with the
decrease in pH. This indicated that if both cooked rice and glucose were used at
high levels ( 10 percent and I percent, respectively) at the beginning of the
fermentation, the pH dropped more slowly than if lower levels were used (8
percent cooked rice and 0.5 percent glucose). Increasing the amount of cooked
rice, on the other hand, reduced the firmness of the nham. There was an increase
in weight loss at the high level of glucose. There were 1.0 to 1.3 percent
reducing sugars and 2 to 3 percent cooked rice in the finished nham, and this
residual carbohydrate could be used by the undesirable organisms during storage.
Therefore, the carbon source levels in nham should be reduced.

When the glucose level was maintained at 0.5 percent but the
level of cooked rice increased, a longer period was required to attain adequate
fermentation end products (16).

It was also found that 6 percent cooked rice with 0.5 percent
glucose in the nham formulation, when fermented with starter cultures at
30°C and 97 percent relative humidity, caused rapid pH reduction. Acid
production was good, firmness and color development were satisfactory, and the
product was microbiologically safe.

The rate of fermentation and the ultimate pH of nham are
directly influenced not only by the specific formulation but also by the
processing conditions. Since the safety and quality of nham depend on the rate
and extent of acid production, a thorough understanding of these environmental
parameters is essential for total control of the product. In our research,
higher temperatures increased the rate of fermentation, reduced pH, and improved
firmness and color development. The initial temperature of nham was very
important in determining the final product. The achievement of lowering pH was
affected by the initial product temperature and the time at that temperature.
For experimentation with frozen meat, the temperature of nham mixtures was
15°C; with fresh meat the temperature was 26°C. The pH dropped more
quickly in nham made with fresh meat than with frozen meat.

Nham made using frozen meat was fermented at 30°C and 97
percent relative humidity. It took 3 days to reduce the pH to 4.3 to 4.4, while
the nham using fresh meat fermented under the same conditions needed only 2 days
to reduce the pH to 4.1.

Nham is usually held at a high temperature during processing to
ensure rapid fermentation, but this can also accentuate the growth of pathogens.
In addition, nham is usually eaten without further cooking by the consumer.
These conditions make strict control of the product essential. Although proper
sanitation, employee hygiene, and the control of raw materials definitely reduce
contamination, ultimate control of product safety must be inherent in the
formulation and process. The addition of starter cultures can provide sufficient
microbial numbers to ensure numerical dominance over the natural flora,
including pathogens, and in combination with the proper processing controls can
guarantee the safety and quality of the final nham.

Shelf Storage

Nham is usually sold in Thai markets at ambient temperatures
(20° to 30°C). It was found that nham prepared using the improved
conditions described here when stored at these temperatures had a shelf life of
9 to 11 days while commercial nham usually has a shelf life of only 3 days. In
supermarkets nham is stored at chilled temperatures (5°C), and it can be
exported at low temperatures (1°C). Additionally, consumers usually store
the product in a household refrigerator (10°C). It was found that shelf
life was extended to 63 to 103 days at storage temperatures of 1° to
10°C. The higher the storage temperature, the greater the change in nham
quality.

Sensory Evaluation

Nham fermented with 103 cfu/g M. varians, 103 cfu/g L.
plantarum, and 106 cfu/g P. cerevisiae with 6 percent cooked rice and 0.5
percent glucose at 30°C, 97 percent relative humidity for 3 days, was
accepted by the trained panel, with an overall acceptability mean ideal ratio
score of 0.95+0.01. For quality degradation during storage, the overall
acceptability of the product depended on sourness and off-flavor detected in the
sample.

Nham using fresh meat fermented at a low temperature was given a
higher than ideal score for sourness. However, the newly developed formulation
for nham was superior to that of the commercial nham.

The consumer panel was also used to determine the effect of
reducing the fermentation time from 3 days to 2 days. The results showed that
only visual texture was significantly different from the ideal product.

In consumer testing the majority of the consumers (90 percent)
accepted the developed nham in terms of sourness, spiciness, and saltiness.

In conclusion, the development of traditional fermented pork
sausage, nham, was very successful in that the product was developed by using
mixed starter cultures and had a very high quality in terms of consistency,
microbiological safety, and longer shelf life. It was also acceptable by the
target consumers. The product could be processed in a simple plant and with
equipment that was available at the fermented meat factory with only an
improvement in the technology of culture preparation and temperature control. In
addition, the developed nham had a longer shelf life than commercial nham. The
product, therefore, could be shipped from the cottage industry producers in the
north to all provinces in Thailand, particularly to Bangkok, and also gave the
potential for overseas shipment if refrigeration is used.

16. Pezacki, W. 1974. Technological control of dry sausage
ripening. VIII. Effect of pre-drying on the dynamics of carbohydrate changes
taking place at the beginning of ripening. Fleischwirtschaft 58:124-126,
129-132,
135.